The present invention relates to an improvement on the apparatus described in U.S. Pat. No. 4,489,432 wherein a method is described for reproducing sound and obtaining a realistic ambient field. One aspect of the method and apparatus described in U.S. Pat. No. 4,489,432, the disclosure of which is hereby incorporated by reference, is the requirement that a left minus right signal be developed and fed to the left sub-speaker and that a right minus left signal be developed and fed to the right sub-speaker. The application of these signals to the sub-speakers in combination with the other aspects of the invention produce an expanded acoustic image and realistic ambient field. However, in one preferred embodiment, the main speaker and sub-speaker on each side are housed within the same acoustic volume. An analysis of the performance of this system reveals the following possible combinations of signals being produced by the main and sub-speaker within the same acoustic volume:
______________________________________ Left Right Main Left Main Right Spkr Sub-Spkr Spkr Sub-Spkr ______________________________________ Left & Right Left Nil Right Nil same Left Only Left left Nil Minus Left Right Only Nil Minus Rt Right Right ______________________________________
From this table it can be seen that although the Main and Sub-speakers occupying the same acoustic volume never work against each other, they are not necessarily working together. It is well known that at midrange frequencies the loading presented by an acoustic volume does not significantly influence the response of the loudspeaker. However, at low frequencies, the response of the loudspeaker is controlled to a great extent by the ratio of total diaphragm area to acoustic volume. Obviously if, as shown in the table above, sometimes one and sometimes two speakers occupying the same acoustic volume may be operating, the ratio of total diaphragm area to acoustic volume will change accordingly by a factor of two depending on the relationship of the right and left channel signals. Under such circumstances it is impossible to tune the system for optimum low frequency response.